The present invention relates to an improved amplifier circuit, and more specifically, to a linear class B transconductance power amplifier suited for driving an inductive load.
In a variety of applications, a transformer winding or other inductive load must be driven linearly with respect to a dynamic voltage signal to produce a magnetic field that tracks the signal waveform exactly. An example of such a requirement is found in the CRT beam convergence systems of color video monitors and television receivers.
Color CRTs of the type used in most present-day television and similar color image display systems have three electrons guns--one for each of three primary colors (red, green and blue). To reproduce color images correctly, the electron beam from these guns must converge at the tube's phosphor display screen, and must remain in convergence as they are deflected over its surface. This is accomplished by bending the beams individually using a combination of fixed and dynamic magnetic fields. In a typical system, magnetic pole pieces mounted on each gun are actuated by individual electromagnets in an external convergence yoke to control the radial positions of the beams.
The convergence circuitry for such a color display system generates suitable correction signals for each beam in synchronization with the horizontal and vertical deflection signals, and drives individual red, green and blue convergence coils in the convergence yoke to maintain the beams in coincidence at all points on the display screen. A total of four dynamic convergence signals may be generated: red, green, blue and blue lateral. Each of these signals is a combination of several correction signals that affect different areas of the screen. The summed correction signals are converted from voltage to current waveforms and applied to the respective convergence coils by individual linear output amplifiers. In a convergence system for a high resolution color CRT, it is important for the output amplifiers to reproduce the correction waveforms accurately, driving each convergence coil with a current that is a linear representation of the input voltage waveform.